Process for the production of free flowing paraformaldehyde



P W HANs-JOACHIM MANN ETAL ,539

PROCESS FOR THE PRODUCTION OF FREE FLOWING PARAFORMALDEHYDE Filed Nov.12, 1964 INVENTORS ATT OR NEYS United States Patent Office 3 ,316,309Patented Apr. 25, 1967 3,316,309 PROCESS FOR THE PRODUCTION OF FREEFLOWING PARAFQALDEHYDE Hans-Joachim Mann, Mainz (Rhine), Walter Pohl,Gotzenhain ulcer Langeu, and Klaus Simon, Buchschlag uber Sprendlingen,Germany, assignors to Deutsche Goldund Silber-Scheideanstalt vormalsRoessler, Frankfurt am Main, Germany Fiied Nov. 12, 1964, Ser. No.410,423 Claims priority, application Germany, Nov. 14, 1963, D 42,931 3Claims. (Cl. 260-6155) The present invention relates to an improvedprocess for the production of free flowing paraformaldehyde and theresultant novel product.

Paraformaldehyde, which is a mixture of polyoxymethylene hydrates of theformula HO(CH O) H 'wherein n can be a number between 8 and 30, isformed by known procedures by the stepwise concentration of aqeuosformaldehyde solutions. The concentrates with formaldehyde contents ofover 70% by weight are water clear liquids at their boiling point, butupon cooling soon solidify to wax-like substances. Further, dehydrationof these hydrogels as well as the transportation, grinding and sievingof the dry products provide considerable technical problems andfurthermore are rather costly with regard toenergy and time. Inaddition, the dry end product is not stable on storage. It agesespecially at temperatures over 20 C. and thereby loses its reactivity.As the free, that is not chemically bound, water contained in theproduct presents an especially good aging inhibitor, an aim in thecommercial production of paraformaldehyde is to leave as much free wateras possible in the end product. However, if such free water content isover 1% by weight the product, in view of its tackiness, can only beconveyed pneumatically with great diificulty and furthermore cannot bepulverized in mills of usual construction. Subsequent moistening of theground, dry pulverized paraformaldehyde not only is expensive but alsodoes not lead to the desired stability against aging in the product.

The handling of the highly concentrated solutions causes difliculties'iuthe production of paraformaldehyde by the concentration of aqueousformaldehyde solutions as, upon cooling, obligomers separate out or theconcentrate solidifies and clogs conduits and valves. Furthermore, asthe hydrogel produced is a poor heat conductor and condenses furtherupon subsequent heating, conduits and valves clogged therewith are noteasily cleared simply by remelting of the solidified hydrogel.

According to the invention it was found that a free flowingparaformaldehyde product could be obtained in a single operation byspraying an aqueous formaldehyde concentrate containing 80 to 90%,preferably about 85 of CH O into a spray drier constructed so as tomaintain a fluidized bed of the spray dried product in the lower portionthereof in which the spray dried product is maintained for a shortperiod of time, preferably 5 to 15 minutes, to effect further drying ofthe product. The product is obtained in the form of small spheres and itpossesses only a relatively slight odor of formaldehyde. In view of thevery small surface area of the spherical particles, they do notdisintegrate upon storage so that the product remains practicallydust-free. The good results obtained by combining the fluidized bed withthe spray drying could not be foreseen. The poor heat conductivity ofthe hydrogel mawe is doubtful whether the heat of condensation liberatedon solidification could be removed with sufficient rapidity. In view ofthe known tackiness of the solidified precondensates it was to beexpected that they would have to be given an after treatment in aseparate apparatus.

The aqueous formaldehyde concentrate is sprayed into the spray drier ata temperature between 105 and 110 C. under a nozzle pressure of 2 to 8kg./cm. At 105 to 110 C. the concentrate is a clear liquid. Thetemperature of the air supplied to maintain the fluidized bed should bebetween about 40 and 60 C. and preferably be 50 C.

The process according to the invention represents a considerabletechnical advance as it is possible therewith to produce a free flowingstable paraformaldehyde product by spraying, with considerable savingsin energy and working time, concentrated aqueous formaldehyde in arelatively simple apparatus. A completely continuous plant for theproduction of free flowing paraformaldehyde can be obtained by theprovision of a continuous evaporator for the concentration of theaqueous formaldehyde supplied to the combined fluidized bed-spray drier.

The accompanying drawing schematically shows an apparatus suitable forcarrying out the process according to the invention.

In carrying out the process according to the invention in the apparatusshown in the drawing, aqueous 40% by weight formaldehyde is supplied tocontinuous evaporator 11 in which it is concentrated under vacuum to anby weight concentrate. The concentrate is sprayed at a temperature of105 C. under a pressure of 3 kg./cm. into the head portion 12 of spraydrier 13. Nozzle 14, conduit 15 and valve 16 are heated. The airnecessary for the fluidized bed is introduced through conduit 17. Theformaldehyde concentrate leaves nozzle 14 in the form of droplets whichsolidify as they drop countercurrently to the warm air which risesthrough the upper widened portion of the apparatus at a velocity ofabout 0.50.7 m./sec. The resulting spherical particles reach thenarrowed lower portion 18 of the apparatus in which the fluidized bed ismaintained in about: 2 seconds. They then remain in such fluidized bedfor about 10 minutes. The upward velocity of the air in the region ofthe fluid ized bed is about 23 m./sec. The exterior of the resultingparticles is sufliciently dry that they leave the apparatus over thesieve plate 19 through discharge means 20 as a free flowing product.

The air which leaves the top of the spray drier 13 through conduit 21 ispassed through condenser 22, and separator 23 in which. the condensedwater vapor and formaldehyde are separated off and is then recycled tothe bottom of the apparatus over blower 24, heat exchanger 25 andconduit 17.

In order that the process according to the invention be carried outsuccessfully it is necessary that the formaldehyde content of theaqueous concentrate supplied to the spray drier be exactly controlled. Aminimum concentration of 80% by weight must be maintained. If a lowerconcentration is employed the droplets formed do not solidify duringtheir passage through the rising warm air and fall down through thedrier as liquids and clog the sieve plate under the fluidized bed zone.Concentrates containing over by weight of formaldehyde can no longer behandled as clear liquid melts.

One or two material nozzles of various constructions can be used as thespray nozzle, preferably a single material nozzle with a bore 5 mm. indiameter is used. The pressure necessary for the spraying can beprovided by a pump of suited construction or by steam or hot air as asecond material. As has already been mentioned, spray pressures of 2 to8 kg./cm. are preferably employed. The optimum pressure depends upon theparticular nozzle used.

The combined fluidized bed-spray drier must be so constructed that theproper velocity gradient of, for example 2-3 down to 0.5-0.7 m./sec. ofthe rising drying air can be exactly maintained. If the velocity is toolow the sieve plate at the bottom becomes obstructed whereas,

.f it is too high, the material which is in the form of fog droplets iscarried out of the top of the spray drier by the air stream.

The temperature of the air introduced into the drier, as has alreadybeen mentioned above, must be between 40 and 60 C. If lower temperaturesare employed suflicient drying of the sprayed material is not attainedand the material leaves the drier while still in a tacky condition. Athigher temperatures the sprayed droplets do not solidify sufficientlyand the sieve plate becomes clogged. Furthermore, considerablequantities of form aldehyde are then contained in the exhaust gases.

The temperatures of the drying air supplied to maintain the fluidizedbed and its upward velocity above the fluidized bed must be so adjustedwith respect to each other that the formaldehyde concentrate dropletssolidify as they drop through the space above the fluidized bed so thatthey are solid when they enter the bed. The upward velocity of thedrying air above the fluidized bed, of course, must be insufficient tosupport the droplets of the concentrate which have been sprayed in sothat they can drop through such rising air into the fluidized bed.

The product obtained according to the invention consists of white,spherical, particles with diameters between about 0.1 and 1.0 mm. Withoptimal spray nozzle adjustment about 75% of the product consists ofparticles With diameters between 0.4 and 0.6 mm. The formaldehydecontent thereof is at least 88% by weight and the free water content isat least by weight. The particles, in view of their dry exteriorsurfaces, are free flowing. On the other hand, the water containing gelcondition in the interior of such particles ensures stability of theparaformaldehyde product on aging. In view of the small surface area ofthe spherical particles they do not disintegrate and the product remainspractically dust free after long periods of storage. Theparaformaldehyde product furthermore, is very reactive and is easilydissolved in water and in phenol. The desirable properties attained inthe product according to the invention provide considerable advantagesfor many intended uses, such as in the production of condensationproducts.

The following examples will serve to illustrate the invention withreference to several embodiments thereof as compared to procedures inwhich the critical concentration and temperature limits were notmaintained. In such examples the proportions are given by weight unlessspecified otherwise.

Example 1 5,000 parts of an aqueous 85.7% formaldehyde concentrate weresprayed into an apparatus as described above at a temperature of 105 C.and a pressure of 3 kg./crn. using a Laval nozzle with a bore 5 mm. indiameter. The temperature of the drying air introduced into the bottomof the apparatus was 50 C. 4500 parts of a free flowing paraformaldehydein the form of spherical particles of an average diameter of 0.4 to 0.6mm. were obtained. The formaldehyde content thereof was 88%. On theother hand, when the process was repeated using a drying air temperatureof 80 C. instead of 50 C. the sprayed droplets did not completelysolidify and a tacky product which only partially was in the form ofspherical particles was obtained. Again, when the same apparatus wasused, except that a spray nozzle with a bore diameter of 3 mm. insteadof 5 mm. was used with a spray pressure of 4 kg./cm. and only a 75%formaldehyde concentrate and a drying air temperature of 30 C. was used,the paraformaldehyde product obtained was in the form of sphericalparticles 0.2 to 2 mm. in diameter. They were very tacky and had to begiven an after treatment.

Example 2 An aqueous 85.7% formaldehyde concentrate was sprayed at atemperature of 105 C. into an apparatus as described above at a pressureof 8 kg./cm. using a nozzle with a bore diameter of 3 mm. Thetemperature of the drying air introduced into the bottom of theapparatus was 60 C. The paraformaldehyde product obtained was in theform of free flowing spherical particles.

We claim:

1. A process for the production of a free flowing paraformaldehydeproduct which comprises spraying a liquid aqueous formaldehydeconcentrate containing to by weight of formaldehyde heated to atemperature between about and C. in the form of droplets into the upperportion of an elongated upright chamber, maintaining a fluidized bed ofsolid particles of the paraformaldehyde product produced in the lowerportion of such chamber, supplying drying air at a temperature between40 and 60 C. and passing it upwardly therethrough, maintaining an upwardvelocity of the drying air in the lower portion of the chambersuflicient to form the fluidized bed of the solid particles of theparaformaldehyde product therein and maintaining an upward velocity ofthe drying air in the upper portion of the chamber which is insuflicientto support the droplets of the formaldehyde concentrate being sprayedin, permitting the droplets to solidify as they drop through the risingdrying air before they reach the fluidized bed maintaining suchsolidified droplets within the fluidized bed for a period of about 5 to15 minutes and then withdrawing the solidified particles from thefluidized bed in a free flowing form.

2. In a process for the production of a free flowing paraformaldehydeproduct which comprises spraying a liquid aqueous formaldehydeconcentrate containing 80 to 90% by weight of formaldehyde in the formof droplets into the upper portion of an elongated upright chamber at atemperature between about 105 and 110 C. and at a pressure between 2 and8 kg./cm. maintaining a fluidized bed of solid particles of theparaformaldehyde product produced in the lower portion of such chamber,supplying drying air at a temperature between 40 and 60 C. and passingit upwardly therethrough, maintaining an upward velocity of the dryingair in the lower portion of the chamber suflicient to form the fluidizedbed of the solid particles of the paraformaldehyde product therein andmaintaining an upward velocity of the drying air in the upper portion ofthe chamber which is insuflicient to support the droplets of theformaldehyde concentrate being sprayed in, the velocity of the dryingair in the lower portion of the chamber in the region of the fluidizedbed being between 2 to 3 m./sec. and 0.5 to 0.7 m./sec. in the upperportion of the chamber, permitting the droplets to solidify as they dropthrough the rising drying air before they reach the fluidized bed,maintaining such solidified droplets within the fluidized bed for aperiod of about 5 to 15 minutes and then withdrawing the solidifiedparticles from the fluidized bed in a free flowing form.

3. The process of claim 2 in which the concentration of the formaldehydeconcentrate is about 85% and the temperature of the drying airintroduced is about 50 C.

References Cited by the Examiner UNITED STATES PATENTS 2,568,017 9/1951MacLean et al. 2606l5.5 2,568,018 9/1951 MacLean et al. 2606l5.52,992,277 7/1961 Porter 260-6l5.5

OTHER REFERENCES Walker, I. F.: Formaldehyde. Reinhold Publishing, NewYork, N.Y., third edition (pp. 145, 148, relied on).

BERNARD HELFIN, Acting Primary Examiner.

LEON ZITVER, Examiner.

H. T. MARS, Assistant Examiner.

1. A PROCESS FOR THE PRODUCTION OF A FREE FLOWING PARAFORMALDEHYDEPRODUCT WHICH COMPRISES SPRAYING A LIQUID AQUEOUS FORMALDEHYDECONCENTRATE CONTAINING 80 TO 90% BY WEIGHT OF FORMALDEHYDE HEATED TO ATEMPERATURE BETWEEN ABOUT 105 AND 110*C. IN THE FORM OF DROPLETS INTOTHE UPPER PORTION OF AN ELONGATED UPRIGHT CHAMBER, MAINTAINING AFLUIDIZED BED OF SOLID PARTICLES OF THE PARAFORMALDEHYDE PRODUCTPRODUCED IN THE LOWER PORTION OF SUCH CHAMBER, SUPPLYING DRYING AIR OF ATEMPERATURE BETWEEN 40 AND 60*C. AND PASSING IT UPWARDLY THERETHROUGH,MAINTAINING AN UPWARD VELOCITY OF THE DRYING AIR IN THE LOWER PORTION OFTHE CHAMBER SUFFICIENT TO FORM THE FLUIDIZED BED OF THE SOLID PARTICLESOF THE PARAMORMALDEHYDE PRODUCT THEREIN AND MAINTAINING AN UPWARDVELOCITY OF THE DRYING AIR IN THE UPPER PORTION OF THE CHAMBER WHICH ISINSUFFICIENT TO SUPPORT THE DROPLETS OF THE FORMALDEHYDE CONCENTRATEBEING SPRAYED IN, PERMITTING THE DROPLETS TO SOLIDIFY AS THEY DROPTHROUGH THE RISING DRYING AIR BEFORE THEY REACH THE FLUIDIZED BEDMAINTAINING SUCH SOLIDIFIED DROPLETS WITHIN THE FLUIDIZED BED FOR APERIOD OF ABOUT 5 TO 15 MINUTES AND THEN WITHDRAWING THE SOLIDIFIEDPARTICLES FROM THE FLUIDIZED BED IN A FREE FLOWING FORM.